It almost looks as though I've jumped rows when I seamed, but I definitely have not.

Here's the pattern directions for the body armhole (left front):
Cast off 5 sts at the beginning of the next row. (42 sts) Work 1 row.
Working all armhole decreases as set by Back [2 stitches in], dec 1 st at armhole edge of next 3 rows, then on foll 2 alt rows, then on 2 foll 4th rows and at the same time dec 1 st at front slope edge of next and foll 6th row, then on every foll 6th row until 26 stitches remain. Cont straight until front matches back to shoulder shaping, ending with RS facing for next row. Cast off 9 sts at beg of next and foll alt row. Work 1 row. Cast off remaining 8 sts. [The only change I made was to work short rows and do a three-needle shoulder bind-off instead of the stair-step cast-off.]

And the directions for the sleeve cap:
Cast off 5 sts at beginning of next 2 rows. Dec 1 st at each end of next 3 rows, then on foll 2 alt rows, then on every foll 4th row until 51 sts remain. Work 1 row, end with RS facing for next row. Dec 1 st at each end of next and every foll alt row to 41 sts, then on foll 5 rows, ending with RS facing for next row (31 sts). Cast off 5 sts at beginning of next two rows. Cast off rem 21 sts.

One LYS employee said she thought that reworking the sleeve cap would fix the issue. (I did the decreases one stitch in; she recommended doing the decreases on the very edge). Another LYS employee thought the sleeve was fine and the issue was with the armhole shaping. Anyone have any thoughts before I starting tearing this thing apart??? I'm totally fine reworking either component (or both) but I want to make sure it's worth it.

Wow, I feel for you! It looks like you may have done one or two many rows on the acutal sleeve itself...is there a possiblity when you were doing increases when you were making the sleeve that you did too many?
I second ztimom about trying to see if there are corrections somewhere for this sweater (which by the way is gorgeous!). When you get done figuring this out can you post the where you found the pattern?
Good luck!
Jen